(1) Field of the Invention
The present invention relates to optical memories for wavelength multiple-type high density recording, and more particularly the invention relates to optochemical hole burning media.
(2) Related Art Statement
Optical recording media in which recorded information can be rewritten to another are broadly classified into the heat mode type and the photon mode type according to the operating principles. In the former, different states: (recorded state/erased state) which are optically discernible from each other are reversely changed by utilizing heating and cooling of the medium with irradiation of laser beam. Magneto-optical media, phase transition media, organic media, etc. belong to this type. In the photon mode type, an intrinsic energy of a light determined by its wavelength is directly used to cause reversible optical changes. Photochromic media and optochemical hole burning (PBB) media belong to this type.
The Persistent Spectral Hole Burning (PSHB) is the phenomenon that when laser beam is irradiated upon a solid in which molecules or ions having optical absorption ability, a hole persistently appears in the spectrum at a wavelength equal to that of the irradiated beam. The hole burning is an effective measure as a high resolution spectroscopy for the solids, and is expected to be applied as a wavelength-multiple type high density optical memory in case that the width (uniform width) of the hole of the hole is smaller than that (non-uniform width) of the absorption spectrum. That is, when the hole burning is effected while the wavelength of the irradiating laser, a plurality of holes independent of one another can be formed in a single spot. If bids of 1 and 0 are made correspondent to the presence and absence of such a hole, the wavelength multiple recording is feasible, so that optical memories at a super high density can be realized. As a material for such an optical memory, materials into which rare earth ions are introduced are known.
However, the media that are at a practical level or a near practical level are of the heat mode type. In any of the optically recording media of the heat mode type, recording is effected by using a single-wavelength light, which poses a limit upon the recording capacity.
On the other hand, the photon mode type is a level of searching fundamental materials. Among the photon mode type optical media, the optochemical hole burning media have the merit that the recording capacity can be greatly increased by overwriting information data at one location at different wavelengths. However, the optochemical hole burning media are still at a level of searching fundamental materials, including the above-mentioned rare earth ion-introduced materials, and materials considered preferable for the optochemical hole burning media are still at a study level. Therefore, materials which can be used for the optochemical hole burning media have been desired to be developed.